1. Field of the Invention
The present invention relates to a deflection scanning apparatus used in a laser beam printer or the like, and more particularly to a deflection scanning apparatus which can prevent noise generation or performance degradation due to vibration during rotation of a rotary polygon mirror therein.
2. Related Background Art
Laser beam printers and laser facsimile machines employ a deflection scanning apparatus, in which a photosensitive drum is scanned with a beam deflected and scanned by a deflector to form an electrostatic latent image thereon. This electrostatic latent image is visualized as a toner image by a developing device and the toner image is transferred onto a record sheet. Subsequently, the record sheet with the transferred toner image is guided through a fixing device to heat-fix the toner on the sheet to complete printing.
FIG. 1 is a plan view to illustrate the structure of a deflection scanning apparatus used in a laser beam printer for scanning a photosensitive member with a beam.
A deflection scanning apparatus 151 is enclosed in a housing 152. FIG. 1 is a plan view of the apparatus in housing 152, a cover of which is removed. The scanning optical apparatus 151 is constituted by a light source 153 including a semiconductor laser device and a collimator lens system, a cylindrical lens 154 for condensing light emitted from the light source 153 into a linear beam, a rotary polygon mirror 155 having a deflection-reflecting surface 155a in the vicinity of a linear image of the beam condensed by the cylindrical lens 154, and an fxcex8 lens 156. The deflection-reflecting surface 155a deflects or reflects the beam. The thus deflected or reflected beam passes through the fxcex8 lens 156 then to impinge on a reflecting mirror 157. The reflecting mirror 157 reflects the beam so that it irradiates a photosensitive drum 158 which is a recording medium.
The rotary polygon mirror 155 has a cross section of a regular hexagon perpendicular to the axis and uniform in the axial direction and has side faces of a reflecting mirror, which constitute the deflection-reflecting surface 155a. The rotary polygon mirror 155 is driven by a motor 159 to rotate at constant speed in the direction of arrow 171 about the axis. The rotation changes with time an angle between an optical path of the beam generated from the light source 153 and then passing through the cylindrical lens 154 and the normal line to the deflection-reflecting surface 155a, which is an incident angle of beam into the deflection-reflecting surface 155a. Since an angle of reflection changes with the change of incident angle, the beam forms a spot on the photosensitive drum 158, moving in the direction of arrow 160 in FIG. 1.
The fxcex8 lens 156 is so designed that the beam reflected on the deflection-reflecting surface 155a is focused to form a spot on the photosensitive drum 158 and that the scanning speed of the spot is kept uniform in the direction of arrow 160. To obtain such characteristics of fxcex8 lens 156, the fxcex8 lens 156 is composed of two lens systems, which are a first fxcex8 lens element 161 and a second fxcex8 lens element 162.
The rotation of rotary polygon mirror 155 in the direction of arrow 171 effects the main scan of beam on the photosensitive drum 158, while the sub-scan is carried out by rotating the photosensitive drum 158 around the axis thereof. An electrostatic latent image is thus formed on the surface of photosensitive drum 158.
Arranged around the photosensitive drum 158 are a corona discharger for uniformly charging the surface of photosensitive drum 158, a developing device for developing the electrostatic latent image formed on the surface of photosensitive drum 158 to form a visual toner image, and a transfer corona discharger for transferring the toner image onto a recording sheet, which are not shown. They work to print the record information according to the beam emitted from the light source 153 on the recording sheet.
A reflecting mirror 173 is provided between the first fxcex8 lens element 161 in fxcex8 lens 156 and the deflection-reflecting surface 155a of rotary polygon mirror 155 and on an optical path L2 in which a beam passes on a more upstream side in the direction of arrow 160 in FIG. 1 than an optical path L1 of a beam reaching a write start position 172 of record information on the surface of photosensitive drum 158. The beam reflected by the reflecting mirror 173 is guided through a condenser lens 174 onto a light receiving surface 175a of light receiving element 175 arranged to include, for example, a photodiode. When the condenser lens 174 focuses the beam deflected and scanned by the rotary polygon mirror 155 such that the beam irradiates the light receiving surface 175a, the light receiving element 175 outputs a signal for detecting a position where the beam is scanned.
The condenser lens 174 and the light receiving element 175 are disposed between the first fxcex8 lens element 161 in fxcex8 lens 156 and the rotary polygon mirror 155, so that an optical path L3 between the reflecting mirror 173 and the condenser lens 174 is located between the first fxcex8 lens element 161 and the rotary polygon mirror 155.
The light source 153 emits a beam in accordance with a signal given from a processing circuit 181 for processing information from a host computer. The signal given to the light source 153 corresponds to information to be written on the photosensitive drum 158, and therefore an electrostatic latent image corresponding to the desired information is formed thereby on the photosensitive drum 158. The processing circuit 181 supplies to the light source 153 a unit of signal representing information corresponding to a scanning line which is a locus of the spot formed by the beam on the surface of photosensitive drum 158. The signal is output in synchronism with the signal given from the light receiving element 175 through a line 176.
A motor 159 is mounted on the bottom of housing 152 and the rotary polygon mirror 155 is attached to a drive shaft 159b of the motor 159. The fxcex8 lens 156 is also mounted on the bottom of housing 152, and the light receiving means arranged to include the reflecting mirror 173, the condenser lens 174 and the light receiving element 175 as described above is set between the rotary polygon mirror 155 and the fxcex8 lens 156.
A conventional deflection scanning apparatus of this type includes a rotary polygon mirror and its driving portion, as shown in FIG. 2, which comprise a base 102 set in a box 101 similar to the above-described housing 152, bearings 103 supported in the base 102, and a shaft 104 rotatably supported by the bearings 103 and in which a drive motor M0 is constituted by a rotor 106 fixed to a flange 105 incorporated with the shaft 104, and a stator 107 fixed to the base 102.
The rotary polygon mirror 108 is urged against the flange 105 by a keep plate 109 screwed on the upper end of shaft 104, a clamping washer 113 and a tension plate 112, as shown in FIG. 2, whereby the rotary polygon mirror 118 is united with the rotor 106 so as to rotate with the shaft 104. The upper opening of box 101 in FIG. 2 is closed by a cover 110.
A light source (not shown), similar to the above-described light source 153, emits a laser beam irradiating the rotary polygon mirror 108 and the laser beam is deflected and scanned with rotation of rotary polygon mirror 108 to advance toward a photosensitive drum (not shown) similar to the above-described photosensitive drum 158. The rotor 106 is provided with a recess 111 for relieving dynamic unbalance caused on the shaft 104 during rotation of the rotary polygon mirror 108. The recess 111 is formed by cutting a part of the surface of rotor 106 before assembling.
The above conventional technology, however, only relieves the dynamic unbalance during rotation of the rotor by removing a part of the rotor before assembling it, and therefore cannot relieve dynamic unbalance arisen from assembly errors in mounting of the rotary polygon mirror or dynamic unbalance caused by unevenness in thickness of members other than the rotor, incorporated with the shaft and rotating therewith, for example, the rotary polygon mirror or the keep plate for keeping the polygon mirror mounted on the shaft.
In more detail, only relieving the dynamic unbalance of the rotor cannot fully reduce the dynamic unbalance caused by the unevenness of thickness, in the direction of rotational axis, of the whole rotating body including the shaft and all members united therewith, which causes vibration of the rotary polygon mirror in rotation at high speed, resulting in noises and degrading the performance greatly.
The present invention permits more precise and speedier balance adjustment, for example, by setting and adhering a plurality of balance weights different in specific gravity into a recess on a rotary polygon mirror.
Specifically, a deflection scanning apparatus comprises a rotation shaft rotatably supported by bearings fixed on a base, a rotor fixed to the shaft, a rotary polygon mirror incorporated with the rotor through elastic press means, and a stator fixed to the base, which is driven to rotate by a motor composed of the shaft, the rotor and the stator, in which at least one member out of the rotor, the elastic press means and the rotary polygon mirror has a concentric recess portion with the center on the center of the rotation shaft and in which a plurality of balance weights different in specific gravity are set in the recess portion to remove rotation unbalance during rotation.
Employing the above arrangement according to the present invention, materials different in specific gravity may be used as the balance weights for relieving the dynamic unbalance of the entire rotating body including the rotary polygon mirror such that a small amount of heavier material is used for first adjustment and another lighter material for second adjustment, whereby more precise balance correction can be made.
Also, the present invention permits more precise and speedier balance adjustment, for example, by providing a plurality of recesses for setting of balance weights on a rotary polygon mirror.
Specifically, a deflection scanning apparatus comprises a rotation shaft rotatably supported by bearings fixed on a base, a rotor fixed to the shaft, a rotary polygon mirror incorporated with the rotor through elastic press means, and a stator fixed to the base, which is driven to rotate by a motor composed of the shaft, the rotor and the stator, in which at least one member out of the rotor, the elastic press means and the rotary polygon mirror is provided with a plurality of concentric recesses with the center on the center of rotation shaft and in which a plurality of balance weights are set in the recesses to remove rotation unbalance during rotation.